The current Framework spec says that the result of intersection is zero or more compatible policy alternatives.  If the intersection fails, there are zero compatible alternatives but there is no way to indicate why the intersection failed.  (If more than one alternative intersected we are in the space of issue "Which Policy Alternative was Selected" http://www.w3.org/Bugs/Public/show_bug.cgi?id=3639 which was marked vNext and "won't fix".)

We have had customers ask for feedback of the form "these policies are not compatible because Assertion X does not appear in both policies" or "these assertions may be compatible but, unfortunately, have different namespaces".  Thus, some form of feedback on why policies are not compatible would  be very useful.

Perhaps this is part of negotiation: http://www.w3.org/Bugs/Public/show_bug.cgi?id=3616